Christian Petersen/Getty Images(CORVALLIS, Ore.) -- Oregon State will be without sophomore quarterback Sean Mannion indefintely after hurting his knee against Washington State, according to head coach Mike Riley.

Mannion threw three interceptions against Washington State on Saturday, a 19-6 Oregon State win.

"It's very, very disappointing for the young man. He's invested a ton and he's going to miss some time," Riley said. "Like I told him, 'You're going to be fine. I read about a guy who hurt his knee in the first game of a the season a few years ago and he's still pretty good -- Tom Brady. The football stuff happens, you've just got to live with it and get better from it.'

Mannion has thrown for over 1,000 yards and seven touchdown for the 10th ranked Beavers, who have the fourth-best offense in the Pac-12.

Junior Cody Vaz will replace Mannion behind center while he is out. Vaz has not seen game action since 2010.